TOKYO (AFP) - Japanese Foreign Minister Fumio Kishida warned on Sunday that "unpredictable events" could follow China's unilateral declaration of an air defence zone over waters including disputed Tokyo-controlled islands.
A Japanese ruling party leader also urged Beijing to help prevent a "touch-and-go situation" after China said on Saturday that it was setting up the "air defence identification zone" over the islands in the East China Sea to guard against "potential air threats".
Mr Kishida told reporters that Japan cannot accept the Chinese measure, which he said was "a one-sided action which warrants us to assume the danger of unpredictable events on the spot".
He added Japan was considering making stronger protests to Beijing "at a higher level".
On Saturday, a division at Japan's foreign ministry lodged a strong protest with a minister at the Chinese embassy in Tokyo by telephone.
Earlier on Sunday, former Japanese foreign minister Masahiko Komura, speaking as deputy head of the ruling Liberal Democratic Party, said Tokyo should "keep on doing what it has been doing in a straightforward manner" apart from making protests to Beijing.
"At the same time, defence officials of the two countries must keep in close communication with each other in order to prevent a touch-and-go situation from arising under any circumstances," Mr Komura said on public broadcaster NHK.
Mr Kishida said the United States is also concerned at China's action.
"We want the countries concerned to have an understanding" on Japan's position, the foreign minister said.
The Japanese Defence Ministry said on Saturday that two Chinese planes entered Japan's own air defence identification zone over the East China Sea, prompting its air defence force to scramble fighter aircraft.
US Secretary of State John Kerry and Secretary of Defence Chuck Hagel said on Saturday they were "deeply concerned" at China's move and committed to defending Japan.
Mr Hagel reiterated that the Senkaku islands - which the Chinese claim and call the Diaoyus - fall under the US-Japan security treaty, meaning that Washington would defend its ally Tokyo if the area is attacked.
The defence chief made it clear that the United States, which stations more than 70,000 troops in Japan and South Korea, would not respect China's declaration of control over the zone.
However, Washington has repeatedly said it has no position on the islands' ultimate sovereignty.
Tokyo last year nationalised some of the islands and has vowed not to cede sovereignty or even to acknowledge a dispute with China.
It accuses its neighbour of trying to change the status quo through intimidation.
China has since sent coastguard vessels and other state-owned ships as well as aircraft close to the islands, sometimes breaching airspace and territorial waters around them.
This has prompted Japanese coastguard boats and air force fighter jets to try to warn them off.
"China used to maintain a certain degree of restraint in the past although it insisted that the Senkaku islands were part of its territory," said Mr Komura, who served as foreign minister from 1998-1999 and from 2007-2008.
"But it is a serious matter that Chinese ships have recently intruded into the territorial waters and that China declared the air defence identification zone this time," he said.